When tracing out track centerlines on a foam surface, I end up drawing a lot of lines perpendicular to one another, to locate curve centers, parallel lines, etc.
To make this easier I found a piece of clear plastic (acetate?), like they print overhead projector presentations on. I taped it to a piece of pegboard and used the peg board holes to locate the points of a right triangle, then used a pin to poke holes in the plastic at the points. I then drew circles around the holes, just to make them easier to see. The triangle was as large as would fit on the plastic.
To use it, I already have one line drawn on the foam. I put a pin through the hole that's at the right angle of the triangle, then push it into the foam into the line where I want the perpendicular to intersect it. I then rotate the plastic around the pin until one of the other holes is also on the line, and put a pin through that hole into the foam as well. Then I use a marker pen with a fine point to mark the foam through the remaining hole, remove the plastic, put a straight edge along that mark and the intersection point, and draw the perpendicular.